audioledoperational-amplifier
I'm working with this design.
I would like to make those LEDs into LED bar graphs that will show the intensity. For example, I want a song with a loud base to make the bar graph shoot up to the top. The bars of the LED bar graph I would like to just be individual LEDs. I was getting the effect with an arduino but I was hoping there was easier way to do this.
I did a seven-channel version of this project a few months ago. I designed 7 separate opamp bandpass filters (using the Bessel filter topology to minimize distortion). I went with the LM3914 and a prepackaged LED bar graph because cost wasn't the issue; development time was, but you can take the opposite route.
I passed the filtered signals through the following peak detector:
This is a lot cheaper than the opamp circuits you'll find elsewhere. The Vce of the transistor and Vf of the diode should cancel out. The 1uF capacitor and 470k resistor gave a good decay rate for watching the audio signals that I was tracking.
With respect to the cost, the LM3914 that you were looking at is just a cascade of comparators with one input connected through a resistor network to a fixed voltage, and the other to the peak detector input. If you don't need the LED current control that this chip provides, you can probably do it cheaper with a classic quad comparator like the LM339 or LM2901 (you don't need anything fancy), which will run you about $0.30 in quantities of 25 (you need 24 for twelve 8-channel graphs). Assuming that resistors are basically free, you need a diode, a transistor, decoupling caps (also essentially free), a 1uF peak storage capacitor, and an LED graph. Just use some bulk 1206 LEDs for the graph and arrange them on your PCB instead of paying for the prepackaged bar graph. If you go with 8 elements in your graph, and you need 12*8 ~= 100 LEDs, you can do that for $0.042 cents apiece with these green indicator LEDs, or about $0.34 per channel. I'd say you can get away with the bar graph part for under $1 per channel if you shop around.
It sounds like you want EL wire. It's basically a thin copper wire wrapped in a phosphor coating, which is surrounded by another thin wire. You apply an AC voltage between the inner and outer wires, and everything starts glowing. It comes in all sorts of colours, it's flexible, and produces a completely continuous line of light.
Best Answer
LM3914 / LM3915 / LM3916 will do this job directly. There are other ICs that will do similarly. Search for "bar graph driver".
These will interface more or less directly in place of the 2N3904 transistors in your circuit. (Just as the SSR's would that I provided references to yesterday). Some minor component changes may be needed.
LM3914 - linear
LM3915 - log response
LM3916 - "extended logarithmic" VU meter type display.
NatSemi product page
Also lists I^2C only (not analog input) LP3944, low voltage, 8 LEDs. LP3943, low voltage , 16 LEDs.
Datasheets:
IIC only - LP3943 product page
Pricing - Digikey, in stock
LM3914 pricing
LM3915 pricing
LM3916 pricing
Note below - display can be a bar or individual dot at selected level.
Edited relevant parts from LM3914 overview. Others similar